Field
This disclosure relates generally to systems and methods for providing an autofocus trigger to an autofocusing system of an imaging device. In particular, the systems and methods provide improved autofocus triggering applicable in still image and video photography.
Description of the Related Art
Autofocus (AF) systems are ubiquitous in imaging devices, including standalone cameras and cameras incorporated into mobile computing devices (for example, cell phones and tablets). The performance of an autofocus system can be characterized in terms of focus and trigger. “Focus” refers to an actual autofocus operation where a lens may be moved to different lens positions to determine the best focus for the current scene. “Trigger” refers to criteria which determine when and/or under what condition an autofocus process should start. Prior approaches to “trigger” functionality include, for example, monitoring a scene in the field-of-view (FOV) of an imaging device, detecting scene changes, and if the scene changes are significant, starting or triggering the autofocus operation. Problems with some current trigger functionality include missing a scene change or false detection of scene changes triggering the autofocus operation too often or too infrequently.
A change in the scene can be a result of a moving object within the scene and/or an imaging device being moved such that it points to a different scene. Some existing autofocus trigger systems monitor metrics, the changes in which can indicate a possibility of a change in a scene requiring triggering a refocusing operation. These metrics can include for example, contrast, luma or exposure, and/or various scene descriptors.
Often the metrics used in prior approaches to triggering autofocus operation are only indirectly or tangentially related to whether or not a refocusing operation is needed. For example, in some approaches a change in exposure is used as a metric to indicate when an AF trigger may be necessary. However, an imaging device which relies on the change in exposure to trigger AF operation, can unnecessarily trigger AF operation when the source of lighting on the object is changed, for example when another object momentarily casts a shadow on the object to be photographed or filmed. Conversely, an AF trigger functionality based on exposure alone can fail to trigger AF operation that might be needed if the object moves position in a way that the exposure is constant while the object is out of focus in the object's new position. Consequently, in some prior approaches, the autofocus operation of an imaging device is triggered unnecessarily, too often or conversely not frequently enough. Accordingly, there is a need for improved systems and methods for determining the timing of triggering of an autofocus operation of an imaging device where the autofocus operation is triggered reliably and when needed.